Ampeg SVT7 Pro

[lowell]

Got another one in here. I fixed one previously on this thread: http://music-electronics-forum.com/t24636/

This one has no speaker output too. This one however seems to be in shutdown mode. I know this because the signal stops at the input to U18A. I believe that the Vactrol U23 controls this with all the other shutdown components... ie Q18. Also the mosfet driver ICs both have 0v on CSD pin 2 which tells me that Q33 is saturated... which is controlled by the shutdown circuitry related to U3. This means the driver ICs are OFF.

U300
pin13: -2vDC
pin15: 3vDC

U17
pin13: 300mvDC
pin15: 19v

So it seems there's something not right about the -2vDC and 3v measurement on the top side of the PA. I've tested the mosfets and don't see anything wrong as I get identical measurements from one side of the PA to the other. Can mosfets be tested like bjt's?

Not quite sure what to look for here. Any advice?

[soundguruman]

The vactrol goes bad for starters. This shuts the amp down. I have seen that occur in another unit.
But it's hard to say, w/ no schematic or (meaningful) measurements. (or can you post the schematics?)
You see, you give voltage readings, but we won't have a clue without the schematics...where that voltage is, in the circuit.
So call LOUD technologies, get the schematic, post it, and then there will be plenty of input I think.

[Jazz P Bass]

Ampeg SVT 7 Schematics

Schematics for SVT7.

[lowell]

The schematics are also in the thread that I referred to as well. If that vactrol fails would this cause the mosfet drivers to be shut off? Finally the vactrol is good with no power, so we know it's not shorted, as I'm getting ~120k across pin 1-2 of U18.

[soundguruman]

That's what I'm trying to see, the schematic is making me seasick.
They invented it, let them fix it?

[soundguruman]

Seriously, it's a Crate amp, not an Ampeg. They started labeling Crate amps as "Ampeg" when they realized that almost nobody would buy a Crate amp.
And Crate, being known for the least reliable amps in the entire industry...has done it again...
Well you can see that this thing has a space age switching power supply, and that there is very little info to go on...very few voltage readings on the schematic.

And as you can see, it went from prototype to production in four months. Then two months after production started, a whole bunch of other parts were added...

So, I would send it back to Loud Technologies. They must know what fails. But I am not so sure that they are going to disclose that information.

Then, learn the lesson. The new Ampeg amps are not really Ampeg. Just because somebody bought the name "ampeg" does not make an Ampeg Amplifier. Like all Crate amps, you can put time and money into fixing it, and you can get it working....and then it breaks again for another reason.

[lowell]

As it often happens, I'm stubborn, and also interested in fixing this thing. I learn alot when I fix amp's I don't understand. LS Electronics is Ampeg tech support. I think they're in MO. Carlos is giving me phone support. I'll report on the findings.

[soundguruman]

You might get it working, but as I said it will just break again.

[Steve Conner]

Soundguruman, why don't you give some constructive advice instead of just banging on about how crap everything is. Your mind is a raging torrent all right, of negativity and random BS.

Having said that, this is a Class-D amp. I'm comfortable working around digital and switchmode things, but high-powered Class-D amps still make me feel uneasy.

At the risk of sounding like a torrent of negativity myself, I've seen (owned!) badly designed Class-D amps that only worked by sheer luck. Until they blew up and then it was obvious the design was just a hopeless case.

The IR gate driver chips in these things have about a half dozen different protection circuits inside. The chip will refuse to drive the MOSFETs unless all of the protection circuits are happy. Maybe by studying the IC datasheet and prodding the pins with a DVM, you can figure out if any of them are unhappy and why. In particular, a blown MOSFET should shut the system down, especially if its gate failed short.

http://www.4qdtec.com/mostest.html shows how to test MOSFETs. Testing them in circuit is harder. When you use the diode test function on a DVM, they should show 0.7Vish with source positive and drain negative, and overrange for all other combinations of pins, unless you charged the gate up while testing it.

Having said that, from a testing perspective, a dead MOSFET looks much like a dead BJT. They only test different when alive.

[soundguruman]

"SVT" stands for Super Vacuum Tube, a real Ampeg built by Magnavox
"SBT" the Super Bass Transistor, Was the actual name for the solid state bass amp built by the real Ampeg.
Now they have put the "SVT" and "Ampeg" label on a transistor amplifier, to make you think you are buying an SVT, which you are not.
You have a switching power supply, with numerous interstage transformers and chokes. A schematic with no clear indication of the actual voltages and waveforms that are supposed to be present.
Understand the difference between the real one and the imitation. Purchase accordingly.

[Steve Conner]

Well I agree for once, it is a cheesy marketing trick to call these modern things "SVT"s.

But that's beside the point, we have a broken amp in need of fixing here, the brand doesn't really matter.

[soundguruman]

It's time to DEMAND that manufacturers follow a much higher standard.
COMPLAIN loud and long. INSIST that these manufacturers provide schematics with concise, accurate servicing information. AND unwavering parts support!
Not just for dealers and service centers, but for ALL of us. I DEMAND this!!!
If the manufacturer can't get with the program, don't buy their products. We send a message and the message is LOUD and CLEAR.

As for amplifiers with "protection circuits", the amp needs to detect high current and shut down BEFORE the parts blow, NOT afterwords.
And here we have another amp that shuts down AFTER the parts blow. Where's the protection??? hahahahahaha the jokes on the customer.

Complain LOUD and long. If you do not complain, things will NEVER get better! Raise the standards NOW.
As far as labeling crate amps "Ampeg" so that people will be fooled into buying them, that's pathetic deception. PU it stinks.

We are far more similar than you realize, we both want reliability, and support, and a great deal more. We are not so different after all.

Perestroika, Comrades.

[lowell]

Thank you Steve. Yes SG you haven't provided much assistance, I agree. I understand your frustration though. At this point I'm gonna replace one of those mosfet drivers as I'm getting some odd voltages around it. That was the problem in the previous one that I fixed so it's a good educated guess. Just havin a hard time finding a supplier.

[lowell]

I'm ordering IRS20957 from mouser. This is listed in IRF notes as an upgraded replacement.

[lowell]

Hey guys just wanna report back the replacing the "100" side mosfet driver did the trick. Both of these amps that have come in with no sound had this same problem.